In air conditioning and refrigeration systems, a compressor is provided to compress a refrigerant and pass that refrigerant through the refrigerant circuit and system components such as a condenser, an evaporator and an expansion device. Scroll compressors and screw compressors are widely used in such air conditioning and refrigerant systems. In both scroll compressors and screw compressors, the refrigerant is compressed as it passes through compression elements associated with a compressor shaft driven in rotation by a drive motor. As the compressor shaft is driven in rotation, the refrigeration passes through progressively smaller compression pockets defining the compression chamber of the compression mechanism. In a screw compressor, the compression mechanism consists of a spiral screw mounted to the compressor shaft and having a screw flight that in association with a surrounding casing defines a progressively compacting compression chamber. In a scroll compressor, the compression mechanism consists of a pair of co-acting scroll members, each scroll member having a generally spiral wrap which interfits with the wrap of the other member to define a compression chamber therebetween. One of the scroll members orbits relative to the other upon rotation of the compressor shaft such that the size of the compression chamber defined between the scroll wraps progressively narrows to compress the refrigerant captured therein.
A shortcoming of such compressors is that, on shutdown, unpowered reverse rotation frequently occurs. It has been general practice to initiate shutdown of the compressor by abruptly terminating electric power to the drive motor. Upon terminating electric power to the motor, the motor no longer applies drive torque to the compressor shaft. Reverse rotation results when compressed refrigerant vapor re-expands from the refrigerant circuit downstream of the compressor discharge port back through the compression chamber to the suction side of the refrigerant circuit upstream of the compressor suction port. As the refrigerant re-expands through the compression chamber, the force of the re-expanding refrigerant drives the unpowered compression mechanism in reverse rotation. The reverse rotation will cease when the pressure between compressor discharge and compressor suction has equalized or nearly equalized.
Such unpowered reverse rotation is undesirable as it can cause damage internal to components of the compressor. Further, unpowered reverse rotation produces an undesirable noise that can be disturbing and annoying to the user of the air conditioning or refrigeration system or can be mistakenly associated with compressor failure. Prior steps to prevent unpowered reverse rotation have generally involved designing an additional component into the compressor such as an internal check valve that closes when the compressed refrigeration vapor begins to re-expand from the compressor discharge back through the compression chamber. When this internal check valve closes, the back flow of the compressed vapor is physically blocked, thus at least minimizing duration of the unpowered reverse rotation or eliminating it. However, the addition of an extra component to the compressor increases the cost of the compressor. Further, the risk exists that the check valve might fail during operation.
Unpowered reverse rotation may also be prevented by including a bypass valve, such as a solenoid or the like, that selectively opens to divert at least a portion of the backflow refrigerant vapor directly to suction thereby bypassing all or at least a portion of the compression mechanism. For example, U.S. Pat. No. 6,042,344 of Lifson discloses a scroll compressor having an unloader bypass valve. At, or shortly before, shutdown, the unloader bypass valve is opened to allow the compressed refrigerant to pass from an intermediate compression stage directly to the compressor suction line, thereby bypassing at least a portion of the compression mechanism. In U.S. Pat. No. 5,167,491, Keller and Chaump disclose a compressor having a dedicated valve installed in a bypass line between the compressor discharge line and the compressor suction line. At, or shortly before, shutdown of the compressor, the valve is opened to allow the compressed refrigerant to pass from the compressor discharge line through the bypass line directly to the compressor suction line, thereby bypassing the compression mechanism altogether. In each of these arrangements, unpowered reverse rotation is thus eliminated or substantially reduced. However, in each of these arrangements, additional components are typically required. Also, some refrigerant may still pass through the compression mechanism.